Semiconductor device

ABSTRACT

An insulating film  103  for making an under insulating layer  104  is formed on a quartz or semiconductor substrate  100 . Recesses  105   a  to  105   d  corresponding to recesses  101   a  to  101   d  of the substrate  100  are formed on the surface of the insulating film  103 . The surface of this insulating film  103  is flattened to form the under insulating layer  104 . By this flattening process, the distance L 1 , L 2 , . . . , Ln between the recesses  106   a,    106   b,    106   d  of the under insulating layer  104  is made 0.3 μm or more, and the depth of the respective recesses is made 10 nm or less. The root-mean-square surface roughness of the surface of the under insulating film  104  is made 0.3 nm or less. By this, in the recesses  106   a,    106   b,    106   d , it can be avoided to block crystal growth of the semiconductor thin film, and crystal grain boundaries can be substantially disappeared.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor thin film formed on asubstrate having an insulating surface and a semiconductor device usingthe thin film as an active layer. Particularly, a thin film obtained bycrystallizing an amorphous semiconductor thin film including silicon asthe main ingredient is used as the semiconductor thin film.

Moreover, the present invention relates to a structure of asemiconductor circuit and an electrooptical device constituted bysemiconductor devices such as thin film transistors, and an electronicequipment provided with them.

In the present specification, it is assumed that all of the foregoingthin film transistor, semiconductor circuit, electrooptical device, andelectronic equipment are included in the category of “semiconductordevice”. That is, any device which can function by using semiconductorcharacteristics will be referred to as a semiconductor device. Thus, thesemiconductor device claimed in the present application includes notonly a single component such as a thin film transistor but also asemiconductor circuit and an electrooptical device made of integrationof such single components, and an electronic equipment provided withthose as parts.

2. Description of the Related Art

In recent years, attention has been paid to a technique for constitutinga thin film transistor (TFT) using a semiconductor thin film (with athickness of about several tens to several hundreds nm) formed on asubstrate having an insulating surface. Development of the thin filmtransistor especially as a switching element for a picture displaydevice (for example, liquid crystal display device) has been hastened.

For example, in the liquid crystal display device, tests have been madeto application of TFTs to any electric circuit, such as a pixel matrixcircuit for individually controlling pixel regions arranged in matrix, adriving circuit for controlling the pixel matrix circuit, and a logiccircuit (arithmetic circuit, memory circuit, clock generator, etc.) forprocessing data signals from the outside.

In the present circumstances, although a TFT using a noncrystallinesilicon film (amorphous silicon film) has been put into practical use, aTFT using a crystalline silicon film (polysilicon film, etc.) isnecessary for an electric circuit requiring performance of further highspeed operation, such as a driving circuit and a logic circuit.

For example, as a method of forming a crystalline silicon film on aglass substrate, there is well known a technique disclosed in JapanesePatent Unexamined Publication No. Hei. 7-130652 and No. Hei. 8-78329 bythe present applicant. According to the technique disclosed in thesepublications, a catalytic element for promoting crystallization of anamorphous silicon film is used to enable the formation of a crystallinesilicon film with excellent crystallinity by a heat treatment at about500 to 600° C. for 4 hours.

Especially, there is also known a technique for obtaining a silicon filmhaving a crystal structure preferable for a device component bynonselectively introducing a catalytic element to control the directionof crystal growth (for example, Japanese Patent Unexamined PublicationNo. Hei. 7-45519 and No. Hei. 8-213634). This technique is called alateral growth method. According to the lateral growth method, crystalgrain boundaries exist parallel to the growth direction, and when thedirection of current of a component is made parallel to the growthdirection, the effect of the grain boundaries can be lowered to thelower most limit. As a result, in spite of polycrystal, characteristicscomparable to a single crystal material can be also obtained.

The lateral growth method will be described in brief. In the lateralgrowth method, a mask film of silicon oxide or the like is formed on anamorphous silicon film, and a window is selectively formed therein. Acatalytic element is introduced through the window by various methodssuch as a sputtering method, a vapor phase growth method, and a coatingmethod.

Then heat crystallization is carried out so that crystal growthprogresses from an introduced portion of the catalytic element as astarting point. This is because the catalytic element crystallizes theamorphous silicon film while diffusing into the silicon film. Ingeneral, as a temperature is high and a time is long, crystallizationprogresses to a further point. The details are disclosed in theforegoing patent publications.

The present inventors have repeated various trials and errors to improvecrystallinity of a crystalline silicon film (called polysilicon film)having crystal grain boundaries. The primary object is to make thecrystal grain boundary substantially harmless. That is, the object is tosubstantially get rid of the crystal grain boundary so that the movementof a carrier (electron or hole) can be made smooth.

The common concept of a semiconductor film disclosed in the foregoingpublications is to make crystal grain boundaries substantially harmless.That is, the primary object was to substantially get rid of the crystalgrain boundaries so that the movement of a carrier (electron or hole) ismade smooth.

Ideally, if higher and longer annealing is carried out, unlimitedlylarge lateral growth ought to be obtained. However, even by asemiconductor film disclosed in the foregoing publications, it wasobserved that although the lateral growth region was enlarged, thequality of crystal was totally lowered. It can be said that the crystalquality is insufficient for high speed operation required by a logiccircuit. That is, in order to realize a system-on-panel having abuilt-in logic circuit, the development of a quite novel material, whichhas not conventionally existed, is desired.

According to the present technique, in the lateral growth method usingnickel as a catalytic element, although the maximum width of lateralgrowth without crystal disturbance is 50 to 60 μm, it is necessary tofurther increase the lateral growth region to enlarge a component.

In crystallization by conventional annealing, it has been consideredthat as the asperities of an under film of a semiconductor thin film arelarge, the occurrence of crystal nuclei becomes easy and a crystalgrowth time becomes short. It is conceivable that this is because theasperity portions function as nuclei of crystallization. However, thepresent inventors found that from various experimental results, in thelateral growth method, since the asperities of an under film reduce thequality of crystal and reduce the speed of lateral growth, it ispreferable that the asperities are as few as possible.

The feature of the lateral growth method is that it is possible toobtain a semiconductor thin film having crystallinity almost equal tosingle crystal by causing only the added catalytic elements to becrystal growth nuclei. However, the asperities of the under film are aptto become crystal growth nuclei, and this crystal growth disturbs thecrystallinity of the lateral growth region and further blocks thelateral growth. Moreover, the asperities generate distortion or the likein the crystal grain, and causes a shift of the line in a crystal axis,or the like. Besides, since the asperities of the under film of thesemiconductor thin film is irregular, there is a fear that crystallinityis different for each place or each substrate.

In general, a quartz substrate is used as a liquid crystal panel, andthe asperities of the quartz substrate were observed by an AFM (AtomicForce Microscope). FIGS. 20A and 20B show AFM photographs. FIG. 20Ashows a quartz substrate generally used for a liquid crystal panel,which is called sample A in this specification. FIG. 20B shows a quartzsubstrate with higher quality than sample A, which is called sample B inthis specification. The respective observed regions are 10×10 μm².

In sample A, although the root-mean-square surface roughness Rms isabout 1 to 1.5 nm, the ten-point-mean roughness Rz is about 10 to 30 nm,and the maximum vertical difference P-V in the observed region is 20 to50 nm. Such vertical difference was observed as a recess caved at asharp angle in a section curve by the AFM of the roughness of sample A.In the photograph shown in FIG. 20A, the recess is confirmed as a blackpoint. The root-mean-square surface roughness Rms is the root of a meanvalue of a square of a deviation from a reference surface to a specifiedsurface.

On the other hand, in the observation photograph of sample B with highquality shown in FIG. 20B, there is no black point such as in FIG. 20A.With respect to the sample B with high quality, the root-mean-squaresurface roughness Rms is about 0.4 to 0.6 nm, the ten-point-meanroughness Rz is about 2 to 4 nm, and the maximum vertical difference P-Vin the observed region is 4 to 9 nm. As indicated by these values, thedepth of a recess of sample B is in an order of several nm, and it isunderstood that sample B has a very flat surface as compared with sampleA.

For example, in the case where a liquid crystal panel is manufactured,as shown in FIG. 19A, an amorphous silicon film 3 is formed on a quartzsubstrate 1 having a recess 2 with a depth 1 to ½ time of the thicknessof an active layer as in sample A and is crystallized. However, as shownin FIG. 19B, since the depth of the recess 2 is almost 1 to ½ time ofthe thickness of the amorphous silicon film 3, a recess 5 reflecting theshape of the recess 2 is also formed on the surface of the amorphoussilicon film 3. When such an amorphous silicon film is crystallized, thecrystal axis is disturbed by this recess 5, so that it is difficult toobtain a film having crystallinity comparable to single crystal.Incidentally, FIG. 19B is an enlarged view showing a vicinity 4 of therecess 2.

Moreover, as shown in FIG. 20A, since the recesses 2 on the surface ofthe quartz substrate 1 are irregularly present, the crystallinity of thecrystalline semiconductor thin film is irregularly disturbed. As aresult, characteristics of individual semiconductor components, such asTFTs, on the same substrate become irregular, so that the reliability isdamaged.

Thus, even if a driving circuit is constituted by such a semiconductorthin film, required performance can not be still completely satisfied.Especially, in the present circumstances, it is impossible to constitutea high speed logic circuit requiring very high speed operation of frommegahertz to gigahertz by conventional TFTs.

The problem due to such a recess can be eliminated if a quartz substratewith high quality, such as sample B shown in FIG. 20B, is used. However,since the substrate becomes expensive, cost performance becomes low.

SUMMARY OF THE INVENTION

An object of the present invention is to enable formation of asemiconductor thin film with crystallinity which can be regarded assubstantially single crystal even if a low grade substrate as shown inFIG. 20A is used.

That is, an object of the present invention is to solve the foregoingproblem and to obtain a high performance semiconductor device such as ahigh speed logic circuit which can not be manufactured by a conventionalTFT, by smoothing and leveling the surface of a substrate on which asemiconductor thin film is formed or the surface of an under film formedon the substrate, to form a semiconductor thin film havingcrystallinity, which can be substantially regarded as single crystal, onthe surface.

Another object of the present invention is to provide a high performancesemiconductor device at low a low price by enabling the formation of asemiconductor thin film having crystallinity which can be substantiallyregarded as single crystal even if a low grade substrate such as sampleA shown in FIG. 20A is used.

In order to achieve the above objects, according to the presentinvention, in a semiconductor device using a semiconductor thin filmmade of a collective of a plurality of rod-like or flattened rod-likecrystals containing silicon as the main ingredient, it is characterizedin that the surface of an insulator as an under film of thesemiconductor thin film has recesses, and the distance between adjacentones of the recesses is not smaller than three times as long as theshort side of the rod-like or flattened rod-like crystals.

According to another aspect of the present invention, in the foregoingsemiconductor device, the distance L between adjacent ones of therecesses is not smaller than 0.3 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are views showing manufacturing steps of an under filmof an embodiment;

FIG. 2 is an enlarged view of a recess of the under film of embodiment1;

FIGS. 3A to 3D are views showing manufacturing steps of the under filmof embodiment 1;

FIGS. 4A to 4E are views showing manufacturing steps of a crystallinesilicon film of embodiment 1;

FIGS. 5A to 5D are views showing manufacturing steps of a semiconductordevice of embodiment 1;

FIGS. 6A and 6B are photographs showing electron diffraction patterns ofsemiconductor thin films;

FIGS. 7A and 7B are views schematically showing an electron beamdiffraction pattern;

FIGS. 8A to 8C are views for explaining the orientation relation of asemiconductor thin film;

FIGS. 9A to 9C are views showing the shapes of crystal grain boundaries;

FIGS. 10A to 10C are views showing manufacturing steps of an under filmof embodiment 3;

FIGS. 11A to 11D are explanatory views of a phosphorus gettering step ofembodiment 5;

FIG. 12 is a view showing a section of an active matrix substrate ofembodiment 6;

FIG. 13 is a view showing an outer appearance of the active matrixsubstrate of embodiment 6;

FIG. 14 is a view showing a section of an active matrix substrate ofembodiment 7;

FIG. 15 is a view showing a section of an active matrix substrate ofembodiment 8;

FIGS. 16A and 16B are views showing examples of semiconductor circuitsof embodiment 9;

FIGS. 17A to 17F are views showing examples of electronic equipments ofembodiment 11;

FIG. 18 is a view showing the measurement result of SIMS;

FIGS. 19A and 19B are sectional views showing a conventional under film;

FIGS. 20A and 20B are atomic force microscope (AMF) photographs ofquartz substrate surfaces;

FIGS. 21A and 21B are TEM photographs showing crystal grains ofcrystalline silicon films;

FIGS. 22A to 22C are views for explaining a model with respect togeneration and disappearance of defects;

FIGS. 23A and 23B are TEM photographs showing crystal grains ofsemiconductor thin films; and

FIGS. 24A and 24B are TEM photographs showing dark field images ofsemiconductor thin films.

DETAILED DESCRIPTION OF THE INVENTION

First, the gist of the present invention will be described withreference to FIGS. 1A and 1B.

FIGS. 1A and 1B are sectional views showing an under insulating layer ofthe present invention, and a semiconductor thin film made of acollective of a plurality of rod-like or flattened rod-like crystalscontaining silicon as the main ingredient is formed on the underinsulating layer.

As shown in FIG. 1A, an insulating film 103 making the under insulatinglayer is formed on a quartz or a semiconductor substrate 100. Recesses105 a to 105 d corresponding to recesses 101 a to 101 d of the substrate100 are formed on the surface of the insulating film 103.

Next, as shown in FIG. 1B, the surface of the insulating film 103 isflattened to form an under insulating layer 104. In this flatteningstep, the recess 105 c is flattened as indicated by 107 to decrease thenumber of recesses 106, so that the distance L1, L2, . . . Ln betweenadjacent recesses 106 is made 0.3 μm or more, preferably 0.5 μm or more.Incidentally, FIG. 2 is an enlarged view showing the vicinity of therecess 106.

The semiconductor thin film of the present invention is one obtained bycrystallizing an amorphous semiconductor thin film. As shown in FIG. 8B,the semiconductor thin film is a collective of a plurality of rod-likeor flattened rod-like crystal grains 251 to 253 containing silicon asthe main ingredient. This rod-like or flattened rod-like crystal grainis crystallized to grow in the <111> direction. This crystal growthdistance (length of a long side) is several tens μm, and on the otherhand, the width w in a short side direction is about 0.1 μm.

In the present invention, by forming the under insulating layer 104 inwhich the distance L between the adjacent recesses 106 is made notsmaller than three times as long as the width w of the short side, thatis, 0.3 μm or more, it is contrived such that the crystal growth of thesemiconductor thin film is not blocked. The distance L between adjacentrecesses 106 is more preferably made not smaller than 5 times as long asthe width w of the short side, that is, 0.5 μm or more. It is indicatedthat as the distance L becomes long, the degree of flatness on thesurface of the insulating layer 104 becomes high. According to thedesign rule of a present submicron device, the channel length is about0.35 μm, and when the distance L is made 0.3 μm or more, it becomespossible to form one channel formation region in an almost flat surface.

It is appropriate that the distance L between the recesses 106 is morepreferably made long up to the degree of a lateral growth distancecorresponding to the length of the long side in the lateral growthdirection, not only in the short side direction. The distance L can bemade 10 μm or more, further, 100 μm by the flattening step.

Further, as shown in FIG. 1B, the number of recesses 106 is reduced bythe flattening step, so that the number (density) per unit area of therecesses 106 can be made 100 pieces/cm² or less. In the presentinvention, in order not to prevent the lateral growth, since the lateralgrowth distance is about 100 μm, if the number per unit area is notlarger than 1÷(100×100 μm²)=10000 pieces/cm², it is conceivable that theinfluence of the recesses 106 of the insulating layer 104 on the lateralgrowth becomes smaller than natural crystallization.

Incidentally, the foregoing distance D and the density of the recessesmay be measured by regarding the places of black points in an AFMphotograph as recesses, as shown in FIGS. 20A and 20B.

However, even if the distance L between the recesses 106 becomes longand the number thereof is made small, if the recesses 106 are deep, thecovering property of the semiconductor thin film at the recess isdamaged and the lateral growth is interrupted by the recess 106. Inorder to solve such a problem, the depth D of the recesses 106 of theinsulating layer 104 is made shallower than the thickness of thesemiconductor thin film by the flattening step. Since the active layerof a TFT has a thickness of about 10 to several hundreds nm, the depth Dis made thinner than the active layer, that is, 10 nm or less,preferably 5 nm or less, more preferably 3 nm or less. Moreover, theroot-mean-square surface roughness Rms of the insulating layer 104 ismade 0.3 nm or less, within the range of 0.3 to 0.2 nm by the flatteningstep. More preferably, the roughness Rms is made 0.15 nm or less. Thesevalues are measured by the AFM.

In this embodiment, when a mean value of the distances L betweenadjacent recesses 106 is La, and a mean value of the depths D of therecesses 106 is Da, Da/La is used as other index of smoothness andflatness. Even if the distance L between the recesses is long, if therecess 105 is deep, a recess is formed on the surface of thesemiconductor thin film formed on the insulating layer 104. Then themean value Da is made smaller than the mean value La. That is, theinsulating layer 104 with such a smooth surface that Da/La is 1 or lessis formed.

As described above, the depth D of the recess is shallower than the filmthickness of the active layer of the TFT and becomes the order of 10 toseveral hundreds nm. On the other hand, the distance L between therecesses becomes the order of 0.1 to 1 μm. Thus, Da/La is made 0.1 orless, preferably 0.01 or less. The lower limit of Da/La becomes about0.001 by the process precision.

In order to form a semiconductor thin film with a good step coverage inthe recess of the insulating layer 104, it is appropriate that the slopeof the side surface of the recess 106 is moderated. Since thesemiconductor thin film has a thickness of about 10 to 100 nm, as shownin FIG. 2, the slope of the side surface is made 50 nm or more in termsof radius of curvature r. From the viewpoint of the step coverage, it ispreferable that the diameter φ of an inlet of the recess 106 is 10 nm ormore, and this condition can be satisfied by the manufacturing steps ofthe following embodiment. The diameter φ of the recess 106 is smallerthan the recess 101 of the substrate and is 1 μm or less.

Preferred embodiments of the present invention will then be described indetail with reference to FIGS. 1 to 18.

Embodiment 1

In this embodiment, explanation will be made to a step of forming asmooth insulating film on a quartz substrate surface, a step of forminga semiconductor thin film having crystallinity regarded as singlecrystal on this insulating film, and a step of manufacturing a TFT usingthe obtained semiconductor thin film.

FIGS. 3A to 3D are explanatory views of manufacturing steps of an underfilm of this embodiment. As shown in FIG. 3A, an inexpensive low gradequartz substrate 100 is prepared. According to the AFM (Atomic ForceMicroscope), although the root-mean-square surface roughness Rms of thequartz substrate 100 is about 1 to 1.5 nm, recesses 101 with a depth of30 to 60 nm and caved at a sharp angle exist irregularly.

Next, as shown in FIG. 3B, an amorphous silicon film 102 is formed onthe quart substrate 100 by a plasma CVD method or a sputtering method.Here, the amorphous silicon film 102 with a thickness of 350 nm isformed by the plasma CVD method. Although not shown here, recesses areformed on the surface of the amorphous silicon film 102 by the recesses101 of the substrate 100, as shown in FIGS. 1 and 2.

Next, as shown in FIG. 3C, the amorphous silicon film 102 is subjectedto thermal oxidation in an atmosphere containing a halide gas (in thisembodiment, HCl) to form a thermal oxidation film 103. It is possible toobtain a thermal oxidation film made denser and having a low interfaciallevel by adding halogen. The thickness of the thermal oxidation film 103is about 700 nm.

FIG. 1A is an enlarged view of FIG. 3C. Since the amorphous silicon film102 has recesses, the recesses 105 a to 105 d corresponding to therecesses 101 a to 101 d are formed on the surface of the thermaloxidation film 103. Thus, if there are such recesses 101 that aninterval between the adjacent recesses 101 of the substrate is 0.3 μm orless, there is a portion where intervals p1, p2, p3, . . . of theadjacent recesses 101 becomes 0.3 μm or less. Further, if there are verydeep recesses 101 a to 101 d, there is a possibility that a recess 105having a depth of 10 nm or more exists in the thermal oxidation film103.

Thus, in this embodiment, as shown in FIG. 3D, flattening of the thermaloxidation film 103 is carried out to flatten the surface of the thermaloxidation film 103 so that the distance between the recesses of theinsulating layer becomes 0.3 μm or more and the depth thereof becomes 10nm or less. FIG. 1B is an enlarged view of FIG. 3D.

As means for flattening, mechanical polishing, chemical mechanicalpolishing (CMP), ELID (Electrolytic In-Process Dressing) and the likemay be used. The ELID is a grinding method using a whetstone formed byfiring cast iron powder mixed with diamond fine grains. A material to bepolished is immersed in a water tank, and the whetstone is made an anodeand water is made a cathode to generate an electric field, so that ironin the whetstone is dissolved and the diamond fine grains appear on thesurface of the whetstone. Thus, setting of the whetstone is alwayscarried out during grinding, so that it is possible to perform mirrorgrinding of a semiconductor material, a glass material, and the like inhigh precision.

In this embodiment, the surface layer of the thermal oxidation film 103is polished to a depth of about 500 nm by the CMP. The remaining thermaloxidation film 103 is the under film 104. Thus, the film thickness ofthe amorphous silicon film 102 is determined in view of the thicknessremoved in the flattening step.

As shown in FIG. 1B, the depth of the recesses 106 a to 106 c of theunder film 104 becomes shallow, and it can be made 10 nm or less,further, 5 nm or less. As shown by the dotted line, a relatively shallowrecess 106 c is made flat by polishing so that the smooth surface 107can be obtained. Thus, since the number of the recesses 105 is reduced,the distance between recesses 106 of the insulating layer 104 issubstantially lengthened, like interval L2, and it became 0.3 μm ormore. The number of the recesses 105 per unit area was made 100pieces/cm² or less.

Moreover, by the flattening step, the root-mean-square roughness Rms wasmade about 0.3 to 0.2 nm, the slope of the side surface of the recess105 was moderated, and the radius of curvature r (see FIG. 2) of theside surface was made 50 nm or more. By moderating the slope, the stepcoverage at the side surface of the recess portion 105 can be improved.By making the radius of curvature r (see FIG. 2) 50 nm or more, it ispossible to form a semiconductor thin film having a thickness of 100 nmto 10 nm with good covering properties.

By the steps shown in FIGS. 3A to 3D, it becomes possible to form theunder film 104 having the smooth surface. After the flattening step,annealing is carried out to eliminate or decrease dislocations anddefects of the surface layer of the under film 104 damaged at thepolishing step. That is, it is important to carry out annealing toremove as many cause, which damages crystal growth of a crystallinesilicon film formed on the under film 104 as possible.

Incidentally, in this embodiment, although the flattening step iscarried out after the thermal oxidation step, it is also possible toform an under film satisfying the conditions of the present invention bycarrying out thermal oxidation after flattening of the amorphous siliconfilm 102.

A method of forming a crystallized semiconductor thin film on the underfilm 104 having such a smooth surface will be described below withreference to FIGS. 4A to 4E. As shown in FIG. 4A, an amorphous siliconfilm 110 is formed on the under film 104. The film thickness of theamorphous silicon film 110 is adjusted so that a final film thickness(film thickness determined after paying consideration to a film decreasesubsequent to thermal oxidation) becomes 10 to 75 nm (preferably 15 to45 nm). In this embodiment, film formation is made by a low pressure CVDmethod and a film is formed under the conditions as follows:

film formation temperature: 465° C.;

film formation pressure: 0.5 torr; and

film formation gas: He (helium) 300 sccm, Si₂H₆ (disilane) 250 sccm.

It is important to thoroughly manage the concentration of impurities ina film at the film formation. In the case of this embodiment, managementis made so that the concentration of each of C (carbon) and N(nitrogen), which are impurities blocking crystallization in theamorphous silicon film 110, becomes less than 5×10¹⁸ atoms/cm³(typically 5×10¹⁷ atoms/cm³ or less, preferably 2×10¹⁷ atoms/cm³ orless), and the concentration of O (oxygen) becomes less than 1.5×10¹⁹atoms/cm³ (typically 1×10¹⁸ atoms/cm³ or less, preferably 5×10¹⁷atoms/cm³ or less) This is because, if the concentration of any one ofthe impurities exceeds the above value, the impurity may have a badinfluence at the time of the subsequent crystallization and may causethe film quality to be degraded after the crystallization.

FIG. 18 shows the result of investigation by SIMS (Secondary Ion MassSpectroscopy) for the concentration of impurities in the amorphoussilicon film manufactured under the conditions of this embodiment. As asample, an amorphous silicon film with a thickness of 0.5 μm formed on asilicon wafer was used. As a result, as shown in FIG. 18, it wasascertained that the concentration of any element of C, N and O waswithin the foregoing range. However, in the present specification, theconcentration of an element in the film is defined as a minimum valueamong measurement results of the SIMS.

In order to obtain the foregoing structure, it is preferable that a lowpressure CVD furnace used in this embodiment is subjected to periodicdry cleaning to clean a film formation chamber. As to the dry cleaning,it is appropriate that the film growth chamber is cleaned by fluorinegenerated by pyrolysis of a ClF₃ (chlorine fluoride) gas flown at a rateof 100 to 300 sccm into the furnace heated to about 200 to 400° C.

According to the knowledge of the present inventors, when thetemperature in the furnace is 300° C. and the flow rate of the ClF₃(chlorine fluoride) gas is 300 sccm, an attachment (containing siliconas the main ingredient) with a thickness of about 2 μm can be completelyremoved in 4 hours.

The concentration of hydrogen in the amorphous silicon film 110 is alsoa very important parameter, and it seems that as the hydrogen content isrestricted to a low value, a film having excellent crystallinity isobtained. Thus, it is preferable that the amorphous silicon film 110 isformed by the low pressure CVD method. Incidentally, if the film growthcondition is optimized, a plasma CVD method may be used.

Next, the amorphous silicon film 110 is crystallized. A techniquedisclosed in Japanese Patent Unexamined Publication No. Hei. 7-130652 bythe present inventor is used as a means for crystallization.

Although both means of embodiment 1 and embodiment 2 disclosed in thepublication may be used, in this embodiment, it is preferable to use thetechnical content (described in detail in Japanese Patent UnexaminedPublication No. Hei. 8-78329) set forth in the embodiment 2 of thepublication.

According to the technique disclosed in Japanese Patent UnexaminedPublication No. Hei. 8-78329, a mask insulating film 111 for selectingan added region of a catalytic element is first formed. Then a solutioncontaining nickel (Ni) as the catalytic element for promoting thecrystallization of the amorphous silicon film 110 is applied by a spincoating method to form a Ni containing layer 112 (FIG. 4A).

As the catalytic element, cobalt (Co), iron (Fe), palladium (Pd),platinum (Pt), copper (Cu), gold (Au), Germanium (Ge), lead (Pb), Indium(In) or the like may be used other than nickel.

An adding step of the foregoing catalytic element is not limited to thespin coating method, but an ion implantation method or a plasma dopingmethod using a resist mask may also be used. In this case, since itbecomes easy to decrease an occupied area of an added region and tocontrol a growth distance of the lateral growth region, the methodbecomes an effective technique when a minute circuit is formed.

Next, after the adding step of the catalytic element is ended,dehydrogenating is carried out at about 450° C. for 1 hour, and then aheat treatment is carried out in an inert gas atmosphere, a hydrogenatmosphere, or an oxygen atmosphere at a temperature of 500 to 700° C.(typically 550 to 650° C.) for 4 to 24 hours to crystallize theamorphous silicon film 110. In this embodiment, a heat treatment iscarried out in a nitrogen atmosphere, at 570° C., and for 14 hours.

At this time, crystallization of the amorphous silicon film 110 proceedswith priority from nuclei produced in a region 113 added with nickel,and a crystal region 114 grown almost parallel to the surface of thesubstrate 100 is formed. The present inventors refer to this crystalregion 114 as a lateral growth region. Since respective crystals in thelateral growth region 114 are gathered in a comparatively uniform state,the lateral growth region has such an advantage that the totalcrystallinity is superior (FIG. 4B).

After the heat treatment for crystallization is ended, as shown in FIG.4C, the mask insulating film 111 is removed, and patterning is carriedout so that an island-like semiconductor layer 116 made of only thelateral growth region 114 is formed. Next, a deposition film 117 makinga gate insulating film is formed. A silicon oxide film, a siliconnitride film, or a silicon nitride oxide film may be formed as thedeposition film 117. It is appropriate that the film thickness isadjusted within the range of 20 to 250 nm in view of an increase of agate oxidation film by a subsequent thermal oxidation step as well. Asthe film formation method, a well-known vapor phase method (plasma CVDmethod, sputtering method, etc.) may be used.

Next, as shown in FIG. 4D, a heat treatment (gettering process for thecatalytic element) for removing or reducing the catalytic element(nickel) is carried out. In this heat treatment, a halogen element ismade contained in a processing atmosphere and the gettering effect for ametallic element by the halogen element is used.

In order to sufficiently obtain the gettering effect by the halogenelement, it is preferable to carry out the above heat treatment at atemperature exceeding 700° C. If the temperature is not higher than 700°C., it becomes difficult to decompose a halogen compound in theprocessing atmosphere, so that there is a fear that the gettering effectcan not be obtained. Thus, the heat treatment is carried out preferablyat a temperature of 800 to 1000° C. (typically 950° C.), and aprocessing time is made 0.1 to 6 hours, typically 0.5 to 1 hour.

As a typical embodiment, it is appropriate that a heat treatment iscarried out at a temperature of 950° C. for 30 minutes in an oxygenatmosphere containing hydrogen chlorine (HCl) of 0.5 to 10 vol % (inthis embodiment, 3 vol %). If the concentration of HCl is equal to orhigher than the above-mentioned concentration, asperities comparable toa film thickness are formed on the surface of the active layer 116.Thus, such a high concentration is not preferable.

As a compound containing a halogen element, one kind or plural kinds ofgases selected from compounds containing a halogen element, such as HF,NF₃, HBr, Cl₂, ClF₃, BCl₃, F₂, and Br₂, may be used other than the HClgas.

In this step, nickel in the island-like region 116 is gettered by theaction of chlorine and is transformed into volatile nickel chloridewhich is released into the air. By this gettering step, theconcentration of nickel in the island-like region 116 is lowered down to5×10¹⁷ atoms/cm³ or less (typically 2×10¹⁷ atoms/cm³ or less). A region120 where the concentration of nickel has been decreased makes an activelayer of a TFT. Incidentally, according to experiences of the presentinventors, when the concentration of nickel is 1×10¹⁸ atoms/cm³ or less(preferably 5×10¹⁷ atoms/cm³ or less), the influence of nickel upon TFTcharacteristics can not be seen.

The above gettering process is also effective for metallic elementsother than nickel. In the step, although constituent elements (typicallyaluminum, iron, chromium, etc.) of the film growth chamber are mainlyregarded as metallic elements having a possibility of mixing into thesilicon film during steps, if the foregoing gettering process is carriedout, it is also possible to make the concentration of those metallicelements 5×10¹⁷ atoms/cm³ or less (preferably 2×10¹⁷ atoms/cm³ or less).When the foregoing gettering process is carried out, the halogen elementthat was used for the gettering process, having a concentration of1×10¹⁵ to 1×10²⁰ atoms/cm³, remains in the island-like region 120.

Moreover, by the above heat treatment, a thermal oxidation reactionprogresses at the interface between the island-like region 116 and theoxide film 117, so that a thermal oxidation film 118 is formed. Theisland-like region 116 not thermally oxidized becomes the active layer120 of a TFT. The thermal oxidation film 118 and the deposition film 117constitute the gate insulating film. When the thermal oxidation film 118is formed after formation of the deposition film 117 in this way, it ispossible to obtain an interface of semiconductor/insulating film whichhas very few-interfacial levels. Moreover, there is also an effect toprevent inferior formation (edge thinning) of the thermal oxidation filmat the end of the active layer 120.

Further, it is also effective that after the heat treatment in theabove-mentioned halogen atmosphere is carried out, a heat treatmentapproximately at 950° C. for one hour is carried out in a nitrogenatmosphere to improve the film quality of the gate insulating film.

Next, a conductive film is formed and patterned to form an original 121of a subsequent gate electrode. In this embodiment, an aluminum filmcontaining scandium of 2 wt % is used. As other films, a tantalum film,a silicon film having conductivity, and the like may be used (FIG. 4E).

Here, a technique disclosed in Japanese Patent Unexamined PublicationNo. Hei. 7-135318 by the present inventors is used. The publicationdiscloses the technique in which source/drain regions and a lowconcentration impurity region are formed in a self-aligning manner byusing an oxide film formed by anodic oxidation. The technique will bedescribed below in brief.

First, as shown in FIG. 5A, an anodic oxidation process is carried outin a solution of 3% oxalic acid while a resist mask (not shown) used forpatterning of the aluminum film is left, so that a porous anodicoxidation film 123 is formed. Since this film thickness becomes thelength of the low concentration impurity region, the film thickness isadjusted to meet the length.

Next, after removing the not-shown resist mask, an anodic oxidationprocess is carried out in an electrolytic solution of an ethylene glycolsolution mixed with tartaric acid of 3%. In this process, a densenonporous anodic oxidation film 124 is formed. It is appropriate thatthe film thickness is 70 to 120 nm.

An aluminum film 125 remaining after the foregoing two anodic oxidationprocesses substantially functions as a gate electrode (FIG. 5A).

Next, the deposition film 117 and the thermal oxidation film 120 areetched by a dry etching method with the gate electrode 125 and theporous anodic oxidation film 123 as masks. Then the porous anodicoxidation film 123 is removed. A gate insulating film 126 is amultilayer made of the deposition film 117 and the thermal oxidationfilm 120, and the end portion of the gate insulating film 126 becomes anexposed state by the thickness of the porous anodic oxidation film 123(FIG. 5B).

Next, an adding step of an impurity element for giving one conductivityis carried out. As the impurity element, it is appropriate that P(phosphorus) or As (arsenic) is used for an N type, and B (boron) or In(indium) is used for a P type.

In this step, a first impurity addition is carried but at a highacceleration voltage to form an n⁻ region. At this time, since theacceleration voltage is as high as 80 KeV, the impurity element is addednot only into the surface of the exposed active layer 120 but also intothe portion under the end portion of the exposed gate insulating film126. Further, a second impurity addition is carried out at a lowacceleration voltage to form an n⁺region. Since the acceleration voltageis as low as 10 keV at this time, the gate insulating film 126 functionsas a mask.

With respect to impurity regions formed through the above describedsteps, the n⁺region becomes a source region 127 and a drain region 128,and the n⁻region becomes a pair of low concentration impurity regions(also called LDD region) 129. A region just under the gate electrode 125is not added with the impurity element and becomes an intrinsic orsubstantially intrinsic channel formation region 130 (FIG. 5C).

After the active layer is completed in the manner described above,activation of the impurity is carried out by the combination of furnaceannealing, laser annealing, lamp annealing and the like, and at the sametime, damages applied to the active layer at the addition steps are alsorepaired.

Next, an interlayer insulating film 131 with a thickness of 500 nm isformed. A silicon oxide film, a silicon nitride film, a silicon nitrideoxide film, an organic resin film, or a lamination film thereof may beused as the interlayer insulating film 131.

Next, after contact holes are formed, a source electrode 132 and a drainelectrode 133 are formed. Finally, the entire substrate is heated in ahydrogen atmosphere at 350° C. for 1 to 2 hours to hydrogenate theentire component so that dangling bonds (unpaired bonds) in the film(especially, in the active layer) are terminated. Through the abovesteps, the TFT having the structure as shown in FIG. 5D can bemanufactured.

Since the main structure of the present invention is a techniquerelating to a semiconductor thin film making an active layer and anunder film of the semiconductor thin film, structure and constitutionother than the main structure do not limit the present invention at all.Thus, the present invention can be easily applied to a TFT havingstructure and constitution other than this embodiment.

Although the silicon film is used as a semiconductor film in thisembodiment, it is also effective to use a silicon film containinggermanium of 1 to 10% as expressed by Si_(X)Ge_(1−X) (0<X<1, preferably0.05≦X≦0.95).

In the case where such a compound semiconductor film is used, thethreshold voltage can be made small when an N-type TFT and a P-type TFTare manufactured. Moreover, the field-effect mobility (called mobility)can be made large.

[Findings as to Crystal Structure of an Active Layer]

The active layer 120 formed in accordance with the manufacturing stepsshown in FIGS. 3 and 4 has microscopically a crystal structure in whicha plurality of rod-like (or flattened rod-like) crystals are arranged inalmost parallel to each other and with regularity to a specificdirection. This can be easily confirmed by observation with a TEM(Transmission Electron Microscope).

FIGS. 23A and 23B are HR-TEM photographs showing crystal grainboundaries of rod-like or flattened rod-like crystals, magnified eightmillion times. In the present specification, the crystal grain boundaryis defined as a grain boundary formed at an interface where rod-like orflattened rod-like crystals are in contact with each other. Thus, thecrystal grain boundary is regarded as different from, for example, amacroscopic grain boundary formed by collision of separate lateralgrowth regions.

Incidentally, the foregoing HR-TEM (High Resolution TransmissionElectron Microscope) is a method in which a sample is verticallyirradiated with an electron beam, and the arrangement of atoms andmolecules is estimated by using interference of transmitted electrons orelastically scattered electrons.

In the HR-TEM, it is possible to observe the state of arrangement ofcrystal lattices as lattice stripes. Thus, by observing the crystalgrain boundary, it is possible to infer the bonding state of atoms inthe crystal grain boundary.

Incidentally, although lattice stripes appear as stripe patterns ofblack and white, they show the difference in contrast, and do not showthe positions of atoms.

FIG. 23A is a typical TEM photograph of a crystalline silicon filmobtained by the present invention, and the state where two differentcrystal grains are in contact with each other at the crystal grainboundary seen from the upper left to lower right in the photograph isobserved. At this time, the two crystal grains were almost in {110}orientation although some deviations were included in crystal axes.

Although to be described later, as the result of investigation of aplurality of crystal grains, it is confirmed by X-ray diffraction orelectron beam diffraction that almost all crystal grains aresubstantially in the {110} orientation. Although many observed crystalgrains ought to include a (011) plane, a (200) plane and the like, thoseequivalent planes will be expressed together by a {110} plane.

As shown in FIG. 23A, lattice stripes corresponding to a {111} plane anda {100} plane are observed in a face. Incidentally, the lattice stripecorresponding to the {111} plane indicates such a lattice stripe thatwhen a crystal grain is cut along the lattice stripe, the {111} planeappears in the section. In a simplified manner, it is possible toconfirm by the distance between the lattice stripes to what plane thelattice stripe corresponds.

The reason why there is a difference in the appearance of the latticestripes in FIG. 23A is that the slopes of the crystal grains are subtlydifferent. That is, when it is designed so that the crystal face of oneof crystal grains is vertically irradiated with an electron beam, theother crystal grain is obliquely irradiated with the electron beam.Thus, the appearance of the lattice stripe is changed.

Here, attention is paid to the lattice stripe corresponding to the {111}plane. In FIG. 23A, the lattice stripe corresponding to the {111} planeof the crystal grain at the upper side above the grain boundaryintersects the lattice stripe corresponding to the {111} plane of thecrystal grain at the lower side at an angle of about 70° (precisely70.5°).

Such crystal structure (precisely, structure of crystal grain boundary)shows that two different crystal grains are in contact with each otherwith extremely excellent conformity. That is, the crystal lattices arecontinuous at the crystal grain boundary so that they become such astructure that it is very hard to produce trap levels due to crystaldefect or the like. In other words, it can be said that the crystallattices have continuity at the crystal grain boundary.

For reference, FIG. 23B shows an HR-TEM photograph of a conventionalhigh-temperature polysilicon film. In the case of FIG. 23B, althoughdescribed later, there was no regularity in the crystal surface, and theorientation was not such that the {110} plane became main. However, forthe comparison to FIG. 23A, such crystal grains that the lattice stripecorresponding to the {111} plane appeared was observed.

When FIG. 23B is observed in detail, as shown by arrows in the drawing,many portions where the lattice stripes are disconnected can beconfirmed in the crystal grain boundary. In such portions, there areuncombined bonds (which can be called crystal defects), and there is ahigh possibility that as trap levels, they block the movement ofcarriers.

However, it is certain that the crystalline silicon film of the presentinvention also includes uncombined bonds as shown in FIG. 23B. This isinevitable as long as the crystalline silicon film of the presentinvention is polycrystal. However, as the result of TEM observation indetail for the crystalline silicon films of the present invention over awide range, it has been found that such uncombined bonds are very few.

As long as the investigation by the present inventors, crystal grainboundaries of 90% or more of the whole (typically 95% or more) were seento have continuity of crystal lattices, and uncombined bonds as shown inFIG. 23B could be hardly found.

Also from this feature, it can be said that the crystal silicon film ofthe present invention is clearly different from conventionalhigh-temperature polysilicon.

FIG. 6A shows the result of investigation of the semiconductor thin filmof the present invention by an electron beam diffraction. For reference,FIG. 6B shows an electron beam diffraction pattern of a conventionalhigh temperature polysilicon film. In FIGS. 6A and 6B, the diameters ofirradiation areas of the electron beams are 4.25 μm and 1.35 μm,respectively. In this embodiment, typical photographs are shown amongmeasurements of plural positions.

In the case of FIG. 6A, diffraction spots corresponding to <110>incidence appear comparatively clearly, and it is ascertained thatalmost all crystal grains are in the {111} orientation within theirradiation area of the electron beam.

The present inventors performed X-ray diffraction in accordance with amethod disclosed in Japanese Patent Unexamined Publication No. Hei.7-321339, and calculated the ratio of orientation with respect to thesemiconductor thin film of the present invention. In the publication,the ratio of orientation is defined with the calculation method asindicated by the following expression 1:{220} orientation existence ratio=1(constant),{111} orientation existence ratio=(relative strength of {111} to {220}of a sample)/(relative strength of {111} to {220} of powder),{311} orientation existence ratio=(relative strength of {311} to {220}of a sample)/(relative strength of {311} to {220} of powder), and{220} orientation ratio=({220} orientation existence ratio)/({220}orientation existence ratio+{111} orientation existence ratio+{311}orientation existence ratio).  [Expression 1]

As the result of investigation of the orientation of the semiconductorthin film of the present invention by the X-ray diffraction, a peakcorresponding to a (220) plane appears. Of course, it is needless to saythat the (220) plane is equivalent to the {110} plane. As the result ofthis measurement, it was found that the {110} plane is the main orientedplane, and the orientation ratio is 0.7 or more (typically 0.9 or more).

On the other hand, in the case of a conventional high temperaturepolysilicon film shown in FIG. 6B, it was found that definite regularitycould not be seen in the diffraction spots, and they were orientedalmost at random, in other words, crystal grains with plane orientationother than the {110} plane were irregularly mixed.

Although the respective diffraction spots have a small concentricexpanse, it is presumed that this is caused by a distribution ofrotation angles of some degree around a crystal axis. This will bedescribed below.

FIG. 7A schematically shows a part of the electron beam diffractionpattern shown in FIG. 6A. In FIG. 7A, a plurality of bright pointsdesignated by 201 are diffraction spots corresponding to the <110>incidence. The plurality of diffraction spots 201 are concentricallydistributed with a center point 202 of the irradiation area as thecenter.

FIG. 7B is an enlarged view showing a region 203 surrounded by a dottedline. As shown in FIG. 7B, when the electron beam diffraction patternshown in FIG. 6A is observed in detail, it is understood that thediffraction spot 201 has an expanse (fluctuation) of about ±1.5° withrespect to the center point 202 of the irradiation area.

That is, it is meant that an angle (corresponding to a half of arotation angle) between a tangential line 204 drawn from the centerpoint 202 of the electron beam irradiation area to the diffraction spot201 and a line connecting the center point 202 of the electron beamirradiation area and a center point 205 of the diffraction spot is 1.5°or less. At this time, since two tangential lines can be drawn, theexpanse of the diffraction spot 201 is eventually within ±1.5°.

This tendency is seen in the entire regions of the electron beamdiffraction pattern shown in FIG. 6A, and in total, the expanse iswithin ±2.5° (typically within ±1.5°, preferably within ±0.5°). Theforegoing statement “the respective diffraction spots have a smallconcentric expanse” means this.

When the under film of the semiconductor thin film is made flat withoutlimit, the ratio (a/b) of the length (a) of a short axis and the length(b) of a long axis of the diffraction spot 201 can be made 1/1 (whichmeans a circle) to 1/1.5. This means that the diffraction spot becomescircular or substantially circular.

In order that the diffraction spot becomes circular, a rotation angleexisting among a plurality of crystal grains must be made very small.When it is considered that a diffraction spot becomes completelycircular in an electron beam diffraction pattern of single crystal, thatthe diffraction spot becomes circular is simply that the semiconductorthin film of the present invention approaches single crystal withoutlimit.

FIGS. 8A to 8C schematically show the relation between the planeorientation of a crystal grain and the crystal axis. FIG. 8A shows therelation between the crystal axis in the case where the planeorientation is {110} and an axis contained in the crystal plane. Likethis, when the crystal plane is in the {110} orientation, the crystalaxis is the <110> axis, and a <111> axis, a <100> axis, and the like areincluded in the crystal plane.

As the result of the previous investigation of grown direction of theforegoing rod-like crystal performed by the present inventors by usingthe HR-TEM, it is ascertained that the rod-like crystal growssubstantially in the direction of the <111> axis (see Japanese PatentUnexamined Publication No. Hei 7-321339). Thus, when a part of thesemiconductor thin film of the present invention is enlarged, it isconceivable that the part is such as shown in FIG. 8B.

In FIG. 8B, reference numerals 251 to 253 denote rod-like crystalsdifferent from each other, and the crystal axes of the respectivecrystal grains are substantially the <110> axes. Since the crystalgrowth on the average progresses substantially toward the direction ofthe <111> axis, the direction of extension of the rod-like crystalsubstantially coincides with the direction of the <111> axis.Incidentally, the portion indicated by a dotted line is a crystal grainboundary.

At this time, when the <111> axis 261 contained in the plane of anarbitrary crystal grain 251 is made a reference axis, <111> axes 262 and263 contained in the planes of other rod-like crystals 252 and 253 thatexist in the vicinity coincide with the reference axis 261 or slightlyshifted so that they have some angle with respect to the reference axis261. In this specification, this angle will be referred to as a rotationangle.

The foregoing statement “the expanse of the diffraction spot is within±2.5° (typically within ±1.5°, preferably within ±0.5°)” is equivalentto, in other words, the statement that the absolute value of therotation angle is within 5° (typically within 3°, preferably 1°).

When this relation is summarized in a simple way as shown in FIG. 8C, inthe semiconductor thin film of the present invention, an angle (α)between the axis 262 and the reference axis 261, and an angle (β)between the axis 263 and the reference axis 261 are rotation angles.This rotation angle is within 5°.

As shown in FIG. 8B, the respective crystal grains having subtlerotation angles appear on the electron beam diffraction pattern asdifferent diffraction spots. For example, the diffraction spots of thecrystal grains 252 and 253 appear on a concentric circle shifted by therotation angles α and β from the diffraction spot of the crystal grain251.

That is, when a plurality of crystal grains exist in the irradiationarea of an electron beam, diffraction spots corresponding to a pluralityof crystal grains are continuously placed on a concentric circle, andthe respective diffraction spots apparently come to show a shape similarto an ellipse. This is the reason why the expanse of the diffractionspot is seen in the electron beam diffraction pattern of FIG. 6A.

Although the notation such as <111> is used in this embodiment, aplurality of equivalent axes such as [111] or [1-11] (the minus symbolmeans inversion) are included therein. That is, the diffraction spotsappear correspondingly to all equivalent axes, and as the result, theelectron beam diffraction pattern as shown in FIG. 6A are formed. Thus,if the crystal grain is rotated by a rotation angle, the electron beamdiffraction pattern is also rotated in the mass by the rotation angle.Accordingly, any diffraction spot has an expanse on the concentriccircle.

As described above, it can be interpreted that the reason why thediffraction pattern as shown in FIG. 6A is obtained as the result ofinvestigation of the semiconductor thin film of the present invention bythe electron beam diffraction is that a plurality of rod-like crystalsexist in the irradiation area of the electron beam and they haverotation angles slightly different from each other. Besides, from thestate of the expanse of the diffraction spot, it is conceivable that theabsolute value of the rotation angle is within 5° (typically within 3°,preferably within 1°).

This means that among all crystal grains constituting the semiconductorthin film of the present invention, even between two crystal grainshaving the largest rotation angles, a shift from an arbitrary referenceaxis is within at least 5°.

Here, in accordance with the category of crystal grain boundariescommonly used, the degree of existence of various crystal grainboundaries in the semiconductor thin film of the present invention willbe described. The table shown below takes data relating to thesemiconductor thin film of the present invention into consideration.

TABLE 1 Semi- conductor thin film of Kind of crystal grain the presentboundary Feature invention Remarks Low angle Small tilt Slight Not existor If these grain grain angle grain rotation with substantiallyboundaries do boundary boundary respect to not exist not exist or(substantially orientation are very few, rotational included in the filmis relation the boundary regarded as within 15 plane single crystaldegrees) (small) Twist Slight or grain rotation with substantiallyboundary respect to single crystal vertical orientation to the boundaryplane Specific high Twin crystal Rotation of Many Since these anglegrain grain 180 degrees (especially grain boundary boundary with respectΣ3) boundaries are to a common electrically orientation inactive, evenOther Common Few if they exist, corre- lattice point the film issponding exists at a regarded as grain fixed ratio single crystalboundary over the or grain substantially boundary to single crystal eachother Random high angle There is no Not exist or If this kind of grainboundary meaningful substantially grain boundary relation of not existexists, the orientation film can not possibly be regarded as singlecrystal

Incidentally, the shape of the crystal grain boundaries shown in Table 1can be distinguished from each other by freely using electron beamdiffraction, HR-TEM, section TEM, and the like, and more detailedinformation can be obtained. Incidentally, the value of the rotationangle in the present specification is measured by analyzing the crystalgrain boundary from various angles by combining the above methods.

Since the foregoing rotation around the crystal axis is “rotation withrespect to orientation included in the boundary plane”, it is containedin the small tilt angle grain boundary. In the case where such a crystalgrain boundary is formed, two crystal grains 271 and 272 are in contactwith each other in the relation schematically shown in FIG. 9A, and havean axis 273 as a rotating axis. In this case, the plane where the twocrystal grains are in contact with each other, is the boundary plane. Inthe semiconductor thin film of the present invention, the rotation anglearound this crystal axis is as very small as ±2.5° or less.

The small tilt angle grain boundary includes also a case as shown inFIG. 9B. In the shape of FIG. 9B, an axis which becomes the rotationaxis 273 is different from FIG. 9A. However, the feature that twocrystal grains 281 and 282 have relation of making a rotation angle 283with the center of an axis included in the boundary plane is similar toFIG. 9A. In the semiconductor thin film of the present invention, sincethe rotation angle in this case is also within ±2.5° (typically within±1.5°, preferably within ±0.5°), it can be considered that such crystalgrains 281 and 282 scarcely exist.

Although it is distinguished from the small tilt angle grain boundaryshown in FIGS. 9A and 9B, there is a shape called a twist grain boundaryin the same category of the low angle grain boundary. This correspondsto the case where a grain is rotated in an orientation vertical to aboundary plane as shown in FIG. 9C.

Also in this case, the feature that two crystal grains 291 and 292 haverelation of making a rotation angle 293 is similar to the small tiltangle grain boundary, and in the semiconductor thin film of the presentinvention, the rotation angle is within ±2.5° (typically within ±1.5°,preferably within ±0.5°). That is, it can be considered that the twistgrain boundary also scarcely exists.

As described above, the semiconductor thin film of the present inventioncan be considered that there is no or substantially no electricallyactive crystal grain boundary, which is generally called a low anglegrain boundary. Incidentally, the words “electrically active” mean thatthe carrier can function as a trap.

Besides, the words “substantially no” mean that for example, whencrystal grain boundaries included in the range of 5 μm square areinvestigated, even if a specified grain boundary (for example, low anglegrain boundary or the like) exists, the number thereof is one or two.

Specific high angle grain boundaries include a twin crystal grainboundary and other corresponding grain boundary, and it is confirmedthat almost all semiconductor thin films of the present invention havethis twin crystal grain boundary. It has been found that even if thecorresponding grain boundary exist, it is electrically inactive (doesnot function as a trap of a carrier).

Especially, in the semiconductor thin film of the present invention, thecorresponding grain boundary of Σ3 ({111} twin crystal grain boundary)occupies 90% or more (typically 95% or more) of the whole, and it isverified in a wide range that crystal grain boundaries with veryexcellent conformity are formed.

Incidentally, the Σ value is a parameter which is an index showing thedegree of conformity of a corresponding grain boundary. It is known thatas the Σ value is small, the conformity of the crystal grain boundary issuperior. The definition of the Σ value is described in detail in “HighResolution Electron Microscope for Estimation of Material; DaisukeSindo, Kenji Hiraga, pp. 54 to 60, Kyoritsu Shuppan Co Ltd., 1996”.

In the crystal grain boundary formed between two crystal grains, whenthe plane orientations of both the crystal grains are {110}, and if anangle formed by lattice stripes corresponding to the {111} plane is θ,it is known that when θ is 70.5°, the boundary becomes the correspondinggrain boundary of Σ3.

Thus, in the crystal grain boundary shown in the TEM photograph of FIG.23A, the respective lattice stripes of adjacent crystal grains arecontinuous at an angle of 70.5°, so that it is easily presumed that thiscrystal grain boundary is the {111} twin crystal grain boundary.

Incidentally, when θ is 38.9°, the boundary becomes the correspondinggrain boundary of Σ9. Such other crystal grain boundary also existedalthough the amount thereof is small.

Such a corresponding grain boundary is formed only between crystalgrains of the same plane orientation. That is, since the planeorientation of the semiconductor thin film of the present invention isuniform as substantially {110}, such a corresponding boundary can beformed over a wide range. This feature can not be obtained by otherpolysilicon films in which the plane orientation is irregular.

The random high angle grain boundary is a grain boundary seen in thesemiconductor thin film which has no meaningful orientation relation andin which crystal grains of irregular orientation are merely lined, andis often seen in a semiconductor thin film such as a conventional hightemperature polysilicon film. In the semiconductor thin film of thepresent invention, it is natural that the high angle grain boundaryhardly exists.

In the case where both the low angle grain boundary and the random highangle grain boundary shown in Table 1 do not exist or the number thereofis very small, it can be considered that active crystal grain boundariesdo not exist. That is, the semiconductor thin film having such a crystalstructure can be regarded as single crystal having substantially nocrystal grain boundary or substantially as single crystal.

As described above, the semiconductor thin film of the present inventionhas orientation relation such that the respective crystal grains(rod-like crystals) constituting the thin film completely coincide inorientation with each other or they have rotation angles of some degree.The rotation angle is as very small as ±2.5° or less, which is on thelevel that a crystal grain boundary is not substantially formed.

The present inventors pay importance to the flatness of an under film asthe reason why such a semiconductor thin film can be obtained. Accordingto the experiences of the present inventors, when the under film hasasperities, they have a large influence on the crystal growth. That is,the asperities or the like of the under film produce distortion or thelike in a crystal grain and causes a shift of a crystal axis and soforth.

The semiconductor thin film of the present invention is formed on theunder film having very high flatness formed by the method described inthis embodiment. Thus, since the film can be grown in the state wherefactors blocking crystal growth are removed to the utmost, crystalgrains are combined with each other while very high crystallinity ismaintained. As a result, it is conceivable that the semiconductor thinfilm having crystallinity, which can be substantially regarded as singlecrystal, can be obtained as described above.

Incidentally, in the formation of the foregoing semiconductor thin film,an annealing step at a temperature above a crystallizing temperature(temperature at the heat treatment carried out in the crystallizing stepof silicon) plays an important role with respect to the lowering ofdefects in a crystal grain. This will be described.

FIG. 21A is a TEM photograph of a crystalline silicon film at a timewhen the steps up to the crystallizing step shown in FIG. 4B have beenended, which is magnified 250 thousands times. Zigzag defects asindicated by arrows are confirmed in the crystal grain (black portionand white portion appear due to difference in contrast).

Although such defects are mainly a lamination defect in which the orderof lamination of atoms on a silicon crystal lattice plane is discrepant,there is also a case of dislocation or the like. It appears that FIG.21A shows the lamination defect having a defect plane parallel to the{111} plane. This can be inferred from the fact that the zigzag defectsare bent at about 70°.

On the other hand, as shown in FIG. 21B, in the crystalline silicon filmof the present invention, which is magnified at the same magnification,it is confirmed that there are hardly seen defects caused by thelamination defect, dislocation and the like, and the crystallinity isvery high.

This tendency can be seen in the entire of the film surface, andalthough it is difficult to reduce the number of defects to zero in thepresent circumstances, it is possible to reduce the number tosubstantially zero.

That is, in the crystalline silicon film shown in FIG. 21B, defects inthe crystal grain are reduced to the degree that the defects can bealmost neglected, and the crystal grain boundary can not become abarrier against movement of carriers due to the high continuity, so thatthe film can be regarded as single crystal or substantially singlecrystal.

Like this, in the crystalline silicon films shown in the photographs ofFIGS. 21A and 21B, although the crystal grain boundaries have almostequal continuity, there is a large difference in the number of defectsin the crystal grains. The reason why the crystalline silicon filmaccording to the present invention shows electrical characteristics muchhigher than the crystalline silicon film shown in FIG. 21A lies mainlyin the difference in the number of defects.

The present inventors consider the following model for a phenomenonoccurring in the step shown in FIG. 4D. First, in the state shown inFIG. 21A, the catalytic element (typically nickel) is segregated at thedefects (mainly lamination defects) in the crystal grain. That is, it isconceivable that there are many bonds having a form such as Si—Ni—Si.

However, when Ni existing in the defects is removed by carrying out thegettering process of the catalytic element, the bond of Si—Ni is cut.Thus, the remaining bond of silicon immediately forms Si—Si bond andbecomes stable. In this way, the defects disappear.

Of course, although it is known that the defects in a crystallinesilicon film disappear by thermal annealing at a high temperature, it ispresumed that in the present invention, since bonds with nickel are cutand many uncombined bonds are generated, recombination of silicon ismore smoothly carried out.

Moreover, at the same time, it is conceivable that surplus silicon atomsproduced at the thermal oxidation of the crystalline silicon film moveto the defects, and greatly contribute to the generation of Si—Si bond.This concept is known as the reason why defects are few in the crystalgrain of the so-called high temperature polysilicon film.

Besides, the present inventors consider a model in which the crystallinesilicon film is bonded to its under film by a heat treatment at atemperature (700 to 1100° C.) above the crystallizing temperature andadhesiveness is increased, so that the defects disappear.

There is a difference of nearly 10 times in thermal expansioncoefficient between the crystalline silicon film and the silicon oxidefilm as the under film. Thus, in the state (FIG. 21A) in which theamorphous silicon film is transformed into the crystalline silicon film,a very large stress is applied to the crystalline silicon film when thecrystalline silicon film is cooled.

This will be described with reference to FIGS. 22A to 22C. FIG. 22Ashows heat hysteresis applied to the crystalline silicon film after thecrystallizing step. First, the crystalline silicon film crystallized ata temperature (t1) is cooled to a room temperature through a coolingperiod (a).

FIG. 22B shows the crystalline silicon film during the cooling period(a), and reference numeral 10 denotes a substrate having an insulatingsurface, and 11 denotes a crystalline silicon film. At this time,adhesiveness at an interface 12 between the crystalline silicon film 11and the substrate 10 is not very high, and it is conceivable that thiscauses production of many defects in grains.

That is, it is conceivable that the crystalline silicon film 11 pulleddue to the difference in the thermal expansion coefficient is veryeasily moved on the substrate 10, so that defects 13 such as laminationdefects and dislocations are easily produced by force such as tensilestress.

The thus obtained crystalline silicon film becomes the state as shown inFIG. 21A. Thereafter, as shown in FIG. 22A, the gettering step of thecatalytic element is carried out at a temperature (t2), and as theresult, the defects in the crystalline silicon film disappear because ofthe above-described reason.

The important point here is that the crystalline silicon film is bondedto the substrate having the insulating surface at the same time as thegettering step of the catalytic element, so that the adhesiveness to thesubstrate is raised. That is, it is conceivable that this gettering stepalso serves as a bonding step of the crystalline silicon film and thesubstrate (under film).

After the gettering+bonding step is ended in this way, the film iscooled to a room temperature through a cooling period (b). Here, thedifferent point from the cooling period (a) after the crystallizing stepis that an interface 15 between the substrate 10 and a crystallinesilicon film 14 after annealing becomes a state having very highadhesiveness (FIG. 22C).

When the adhesiveness is high like this, since the crystalline siliconfilm 14 is completely bonded to the substrate 10, even if stress isapplied to the crystalline silicon film at the cooling step of thecrystalline silicon film, defects are not produced. That is, it ispossible to prevent defects from being produced again.

Incidentally, in FIG. 22A, although the process in which the temperatureis lowered to a room temperature after the crystallizing step isexemplified, it is also possible to carry out the gettering+bonding stepby directly raising the temperature after the end of crystallization.Even if such a process is carried out, the crystalline silicon film ofthe present invention can be obtained.

The thus obtained crystalline silicon film (FIG. 21B) of the presentinvention has a feature that the number of defects in the crystal grainsis greatly smaller than the crystalline silicon film (FIG. 21A) in whichmerely crystallization is carried out.

The difference in the number of defects appears as the difference inspin density by an electron spin resonance analysis (Electron SpinResonance: ESR). In the present circumstances, it has been ascertainedthat the spin density of the crystalline silicon film of the presentinvention is at most 5×10¹⁷ spins/cm³ or less (preferably 3×10¹⁷spins/cm³ or less). However, since this measurement value is near thedetection limit of an existing measuring device, it is expected that theactual spin density is lower than this value.

The present applicant refers to the crystalline silicon film having theabove described crystal structure and features according to the presentinvention as a continuous grain boundary crystalline silicon (ContinuousGrain Silicon: CGS). (Findings as to corresponding grain boundary)

The corresponding grain boundary described before is formed only betweencrystal grains having the same plane orientation. That is, since theplane orientation of the semiconductor thin film of the presentinvention is uniform in orientation substantially {110}, such acorresponding grain boundary can be formed over a wide range. Thisfeature can not be obtained by other polysilicon film having irregularplane orientation.

FIG. 24A is a TEM photograph (dark field image) of a semiconductor thinfilm of the present invention, which is magnified 15 thousands times.Although white regions and black regions are seen in the photograph, itis shown that portions of the same color have the same orientation.

The remarkable feature in FIG. 24A is that in the dark field image ofsuch a wide range, the white regions are continuously clustered at arather high rate. This means that crystal grains having the sameorientation exist with some directionality, and adjacent crystal grainshave almost the same orientation.

On the other hand, FIG. 24B is a TEN photograph (dark field image) inwhich a conventional high temperature polysilicon film is magnified 15thousands times. In the conventional high temperature polysilicon film,portions of the same plane orientation merely exist at random, and theuniformity with directionality as shown in FIG. 24A can not beconfirmed. It is conceivable that this is caused from completeirregularity of orientation of adjacent crystal grains.

By repeating observations and measurements over a number of regionsother than the measured points shown here, the present applicant hasconfirmed that the continuity of the crystal lattices at the crystalgrain boundary is maintained in a sufficiently wide region formanufacturing a TFT. From these results of observation, it was confirmedthat continuity of the crystal lattice in an arbitrary crystal grainboundary was maintained, and a planar grain boundary was formed.

Embodiment 2

In the embodiment 1, although the thermal oxidation film formed on thequartz substrate 100 is used as the under film of the semiconductor thinfilm, it is also possible to use a thermal oxidation film obtained bythermal oxidation of a surface of a silicon substrate. Also in thiscase, it is preferable that the thermal oxidation step of the surface ofthe silicon substrate is carried out in an atmosphere containing halogensuch as chlorine since a denser oxide film can be obtained.

When the surface of the silicon substrate is subjected to thermaloxidation and the flattening process such as CMP is applied to thethermal oxidation film, it is possible to obtain an under insulatingfilm having a root-mean-square surface roughness Rms of 0.3 nm or less,and a recess with a depth of 10 nm or less. Moreover, in this underinsulating film, it is possible to make the number of recesses per unitarea 100 pieces/cm² or less, and the distance between adjacent recesses0.3 μm or more.

Embodiment 3

In the embodiment 1, although the thermal oxidation film formed on thequartz substrate 100 is used as the under film of the semiconductor thinfilm, in this embodiment, a deposition film formed by a CVD method orthe like is used. This embodiment will be described with reference toFIGS. 10A to 10C.

As shown in FIG. 10A, an inexpensive low grade quartz substrate 800 isprepared. Although the surface of the quartz substrate 800 is almostflat, there are irregularly recesses 801 caved at a sharp angle.

As shown in FIG. 10B, a deposition film 802 made of silicon oxide,silicon nitride, or silicon nitride oxide is formed on the quartzsubstrate 800 by a plasma CVD method or a sputtering method. Here, asilicon oxide film with a thickness of 500 nm is formed by the plasmaCVD method. There are recesses on the surface of the deposition film 802due to the recesses 801.

Next, as shown in FIG. 10C, the deposition film 802 is flattened bymechanical polishing, chemical mechanical polishing (CMP), ELID(Electrolytic In-Process Dressing), or the like. The mechanicalpolishing, chemical mechanical polishing (CMP), ELID (ElectrolyticIn-Process Dressing), or the like may be used as means for flattening.Annealing is carried out after the flattening step so that thecrystallinity of an under film 803 damaged by the polishing step isimproved.

In this embodiment, the surface layer of the deposition film 802 ispolished and removed by the CMP by a thickness of about 300 nm. Theremaining deposition film 802 is the under film 803. By the flattening,it is possible to make such a state that the interval between therecesses on the under film 803 is 0.3 μm or more, and the depth thereofis 10 nm or less.

Moreover, the root-mean-square surface roughness Rms of the under film803 can be made 0.3 nm or less, and the number of the recesses per unitarea can be made 100 pieces/cm² or less.

Then, in accordance with the method described in the embodiment 1, acrystallized silicon thin film is formed on the under film 803 havingthe smooth surface. Since the under film 803 is made smooth, it ispossible to obtain a semiconductor thin film which can be almostregarded as single crystal described in the embodiment 1.

Embodiment 4

In the embodiments 1 and 3, explanation has been made to the method inwhich an insulating film is formed on a quartz substrate, and recesseson the surface of the substrate are absorbed to obtain a smoothinsulating surface. In this embodiment, a case of polishing the surfaceof a quartz substrate will be described.

First, a quartz substrate is prepared. Next, the surface layer of thequartz substrate is polished by a means such as CMP (Chemical MechanicalPolish) to flatten the surface. After the flattening step is ended, thequartz substrate is annealed to improve the crystallinity.

As a result, the recess becomes shallow, the root-mean-square surfaceroughness Rms of the quartz substrate can be made 0.5 nm or less,further, 0.3 nm or less, and the maximum vertical difference P−V can bemade 10 nm or less, about 5 to 3 nm. There is a portion where the recessobserved in FIG. 20A is not observed in the range of 10×10 μm², thedistance L between adjacent recesses can be made 0.3 μm or more, and thenumber of the recesses can be made 10000 pieces/cm² or less, further,100 pieces/cm².

As described above, even the inexpensive quartz substrate can be used asan insulating substrate having excellent flatness by polishing. When thequartz substrate is used, the under film becomes very dense, so that thestability of an interface between the under film and semiconductor thinfilm is high. Moreover, since there is scarcely any influence ofpollution from the substrate, the usefulness is very high.

Besides, when an insulating under film is formed on this flattened andsmoothed quartz substrate by the method described in the embodiments 1and 3, it becomes possible to obtain the surface with superior flatness.

Embodiment 5

In the embodiment 1, there has been shown an example in which thehalogen element is used in the step of gettering the catalytic elementfor promoting crystallization of silicon. In the present invention, itis also possible to use a phosphorus element in the gettering step ofthe catalytic element.

First, in accordance with the method described in the embodiment 1, thestructure shown in FIG. 4B is obtained. Next, as shown in FIG. 11A,after the mask 111 is removed, a resist mask 150 covering at least aregion which becomes an active layer is formed.

As shown in FIG. 11A, by using the resist mask 150, phosphorus is addedinto regions other than the region that becomes an active layer, so thata gettering region 151 is formed. As the adding method, a vapor phasemethod such as an ion doping method, a liquid phase method such as aspin coating method, a sputtering method of a film containingphosphorus, and a solid phase method using formation by a CVD method maybe used. The lateral growth region 114 where phosphorus has not beenadded will be referred to as a gettered region 152.

As shown in FIG. 11A, after the resist mask 150 is removed, it isappropriate that a heat treatment is carried out at a temperature of 400to 1050° C. (preferably 600 to 750° C.) for 1 min to 20 hr (typically 30min to 3 hr). The catalytic element is gettered by this heat treatmentinto the region added with phosphorus, so that the concentration of thecatalytic element in the gettered region 152 is reduced down to 5×10¹⁷atoms/cm³ or less.

After the gettering step is ended in this way, the gettered region 152is patterned to form an active layer 153. Subsequently, if the steps inthe embodiment 1 are carried out, a semiconductor device having the samefeature as the embodiment 1 can be obtained.

Of course, if a heat treatment is carried out in an atmospherecontaining a halogen element at the formation of a thermal oxidationfilm which becomes a gate insulating film, the synergistic effect of thegettering effect due to the phosphorus element in this embodiment andthe gettering effect due to the halogen element can be obtained.

Embodiment 6

In this embodiment, an example in which a reflection type liquid crystalpanel is constituted by using a semiconductor device shown in theembodiment 1 will be described. FIG. 12 is a sectional view showing anactive matrix type liquid crystal panel, which shows a CMOS circuit in aregion where a driver circuit or a logic circuit is constituted, and apixel TFT in a region where a pixel matrix circuit is constituted. Thedriver circuit, logic circuit, and pixel matrix circuit are manufacturedon an under film 301 formed on a quartz substrate 300. The under film301 is manufactured by the method described in the embodiments 1 to 4.

The CMOS circuit is fabricated by complementarily combining an N-channelTFT and a P-channel TFT. Since the structure and manufacturing method ofthe respective TFTs constituting the CMOS circuit have been described inthe embodiment 1, the explanation will be omitted.

In the pixel TFT, it is necessary to further make contrivance to a TFTconstituting the driver circuit and the like. In FIG. 12, referencenumeral 303 denotes a silicon nitride film, which serves also as apassivation film of the CMOS circuit, and at the same time, whichfunctions as an insulator constituting auxiliary capacitance.

A titanium film 304 is formed on the silicon nitride film 303, and theauxiliary capacitance is formed between the titanium film 304 and adrain electrode 305. At this time, since the insulator is the siliconnitride film 303 having high relative dielectric constant, thecapacitance can be made large. Since it is not necessary to payconsideration to an opening rate in the reflection type, there is noproblem even if the structure as shown in FIG. 12 is made.

Next, reference numeral 306 denotes an interlayer insulating film madeof an organic resin film, and acryl is used in this embodiment. It ispreferable to make the film thickness of the interlayer insulating filmas thick as about 2 μm and to secure sufficient flatness. By doing so,it is possible to form a pixel electrode 307 having excellent flatness.

A pixel electrode 307 is made of aluminum or a material containingaluminum as the main ingredient. It is preferable to use a materialhaving reflectivity as high as possible. When excellent flatness issecured, it is possible to decrease loss by diffused reflection on thesurface of the pixel electrode.

An oriented film 308 is formed on the pixel electrode 307. A groove isformed in the oriented film 308 by rubbing. The above is the explanationas to the structure of the TFT side substrate (active matrix substrate).

On the other hand, an opposite side substrate is constructed by forminga transparent conductive film 310 and an oriented film 311 on atransparent substrate 309. Other than those, a black mask or a colorfilter may be provided as the need rises.

After spacers are distributed and sealing materials are printed, aliquid crystal layer 312 is included so that a reflection type liquidcrystal panel with a structure as shown in FIG. 12 is completed. Theliquid crystal layer 312 can be freely selected by an operation mode(ECB mode, guest and host mode, etc.) of liquid crystal.

FIG. 13 schematically shows the outer appearance of the active matrixsubstrate constituting the reflection type liquid crystal panel as shownin FIG. 12. In FIG. 13, reference numeral 300 denotes a quartzsubstrate, and in accordance with the steps of the embodiment 1, anunder film 301 made of a thermal oxidation film is formed on the quartzsubstrate 300. A pixel matrix circuit 404, a source driver circuit 405,a gate driver circuit 406, and a logic circuit 407 are arranged on theunder film 301.

Although the logic circuit 407 contains all logical circuits constitutedby TFTs in a wide sense, in order to distinguish it from circuitsconventionally called a pixel matrix circuit and a driver circuit, thelogic circuit in this specification means a signal processing circuit(memory, D/A converter, clock generator, etc.) other than those.

An FPC (Flexible Print Circuit) terminal as an external terminal isattached to the thus formed liquid crystal panel. In general, what iscalled a liquid crystal module is a liquid crystal panel in the state inwhich the FPC is attached thereto.

Embodiment 7

In this embodiment, an example in which a transmission type liquidcrystal panel is constructed by using a semiconductor device shown inthe embodiment 1, will be shown in FIG. 14. The transmission type panelis manufactured on a substrate 500 on which an under film 501 explainedin the embodiment 1 or 3 is formed. Since the basic structure of thepanel is the same as the reflection type liquid crystal panel shown inthe embodiment 4, differences in structure will be especially described.

In the case of the transmission type liquid crystal panel shown in FIG.14, the structure of a black mask 503 is greatly different from thereflection type liquid crystal panel. That is, since an opening ratemust be secured in the transmission type, it is important to make such astructure that the black mask 503 does not overlap, to the utmost, at aportion other than a TFT portion and a wiring portion.

Thus, a drain electrode 504 is formed to overlap with the TFT portion,and auxiliary capacitance is formed between the electrode and a blackmask 505. In this way, it is possible to widen the opening rate byforming the auxiliary capacitance, which is apt to occupy a wide area,over the TFT.

Reference numeral 505 denotes a transparent conductive film whichbecomes a pixel electrode. Although ITO is most frequently used for thetransparent conductive film 505, other materials (tin oxide basedmaterial, etc.) may be used.

Embodiment 8

In this embodiment, the present invention is applied to a so-calledsilicon gate TFT in which a conductive silicon film is used as a gateelectrode. A panel in this embodiment is manufactured on a substrate 600on which an under film 601 explained in the embodiment 1 or 3 is formed.Since the basic structure is almost the same as the TFT manufactured inthe embodiment 1, explanation will be made while paying attention toonly the differences.

In FIG. 15, reference numeral 603 denotes a gate electrode of anN-channel TFT, 604 denotes a gate electrode of a P-channel TFT, and 605denotes a gate electrode of a pixel TFT. An N-type polysilicon filmadded with phosphorus or arsenic, or a P-type polysilicon film addedwith boron or indium is used for gate electrodes 603 to 605.

In the CMOS circuit, a dual gate type CMOS circuit in which an N-typepolysilicon gate is used for an N-channel TFT and a P-type polysilicongate is used for a P channel TFT may be constituted.

Like this, as merits of using the silicon film for the gate electrode,there are enumerated such features that the heat resistance is high, andhandling is easy because of the silicon film. A salicide structure(including polycide structure as well) can be adopted by using reactionwith a metallic film.

For that purpose, side walls 606 to 608 are formed after the gateelectrodes 603 to 605 are formed, metallic films (not shown) oftitanium, tungsten or the like are formed, and a heat treatment iscarried out to form metal silicide 609 to 611. The metal silicides 609to 611 are formed in source/drain regions and part of the gateelectrodes.

Like this, the structure in which metal silicide is formed in aself-aligning manner by using a side wall or the like is called asalicide structure. When such a structure is adopted, since the ohmiccontact to a leading electrode (source/drain electrodes, etc.) becomesexcellent, it is effective.

Embodiment 9

Since the TFT of the present invention uses a semiconductor thin film,which can be substantially regarded as single crystal, as an activelayer, the TFT shows electrical characteristics comparable to a MOSFETusing single crystal. Data as set forth below are obtained from TFTsexperimentally formed by the present inventors.

(1) The subthreshold coefficient as an index showing switchingperformance (promptness in switching of on/off operation) of a TFT is assmall as 60 to 100 mV/decade (typically 60 to 85 mV/decade) for both anN-channel TFT and a P-channel TFT.

(2) The field effect mobility (μ_(FE)) as an index showing an operationspeed of a TFT is as large as 200 to 650 cm²/Vs (typically 250 to 300cm²/Vs) for an N-channel TFT, and 100 to 300 cm²/Vs (typically 150 to200 cm²/Vs) for a P-channel TFT.

(3) The threshold voltage (V_(th)) as an index indicating a drivingvoltage of a TFT is as small as −0.5 to 1.5 V for an N-channel TFT and−1.5 to 0.5 V for a P-channel TFT.

As described above, the TFT obtained in the present invention hasextremely superior switching characteristics and high speed operationcharacteristics. Thus, it becomes possible to constitute an integratedcircuit such as an LSI, which is conventionally constituted by MOSFETs,by TFTs.

Further, it becomes also possible to constitute a semiconductor device(semiconductor circuit) of a three-dimensional structure by effectivelyusing the merit of a TFT using a thin film.

FIGS. 16A and 16B show examples of semiconductor circuits ofthree-dimensional structure using TFTs of the present invention. FIG.16A shows a three-dimensional circuit in which a TFT layer at a lowerside and an image sensor at an upper side are laminated. FIG. 16B showsa three-dimensional circuit in which TFT layers are laminated at upperand lower layers, and the TFT layer at the lower side is manufactured ona quartz or silicon substrate 700 on which an under film 701 explainedin the embodiments 1 to 4 is formed.

In FIG. 16A, reference numeral 703 denotes a photoelectric conversionlayer to which an amorphous silicon film or the like may be used. Anupper electrode (transparent conductive film) 704 is disposed thereon,which constitutes a light receiving portion receiving light andconverting it into an electric signal.

Since the manufacturing steps of the TFT have been described in theembodiment 1, the explanation will be omitted. A well-known means may beused as a laminating technique for constituting the three-dimensionalcircuit. However, in the case where the upper TFT layer is formed, it isnecessary to take the heat resistance of the lower TFT intoconsideration.

For example, it is also acceptable to adopt such a structure that thelower layer is constituted by a TFT of the present invention and theupper layer is constituted by a conventional TFT formed at a lowtemperature. Also, the lower layer may be formed of a material with highheat resistance, and the upper layer may be formed of a TFT of thepresent invention.

Besides, it is also acceptable to adopt such a structure that an imagesensor which becomes an upper layer is constituted by only a lightreceiving portion, and a TFT in a lower layer controls the lightreceiving portion in the upper layer.

Next, in FIG. 16B, the lower layer is a TFT layer using a silicon gatestructure, and the upper layer is a TFT layer with a silicon gatestructure or a structure in which other metallic film (film containingaluminum as the main ingredient, etc.) is used as a gate electrode. Alsoin FIG. 16B, the explanation of the TFT structure will be omitted.

Even in such a structure, it is necessary to manufacture the upper TFTafter paying sufficient consideration to the heat resistance of thelower TFT.

Moreover, in both of FIGS. 16A and 16B, it is desirable that interlayerinsulating films 705 and 706 with a sufficient thickness are formedafter the formation of the TFT in the lower layer, and the films arepolished by the CMP (Chemical Mechanical Polishing) or the like toflatten, and then the upper TFT is formed.

As described above, when a semiconductor circuit of a three-dimensionalstructure is constituted by using the TFT of the present invention, itis possible to constitute a semiconductor circuit having very excellentfunctionality. Incidentally, in the present specification, the term“semiconductor circuit” is used to mean an electric circuit in whichcontrol and conversion of electric signals is carried out by usingsemiconductor characteristics.

It is also possible to constitute a high frequency circuit (MMIC:Microwave Module IC) or the like for an LCD driver circuit or a portableequipment by using the TFT of the present invention. That is, by usingthe TFT of the present invention, it is possible to manufacture aconventional IC chip or an LSI chip by using the TFT of the presentinvention.

Embodiment 10

In the present invention, other than the liquid crystal display device,it is also possible to manufacture other electrooptical devices such asan active matrix type EL (electroluminescence) display device or an EC(electrochromics) display device. It is also possible to manufacture animage sensor or a CCD.

Incidentally, the term “electrooptical device” is used to mean a devicefor converting an electric signal into an optical signal or a device forperforming vice versa.

Embodiment 11

In this embodiment, examples of electronic equipments (applied products)which utilize an electrooptical device using the present invention willbe described with reference to FIGS. 17A to 17F. Incidentally, theelectronic equipment means a product provided with a semiconductorcircuit and/or an electrooptical device.

As the electronic equipments to which the present invention can beapplied, a video camera, an electric still camera, a projector, a headmount display, a car navigation system, a personal computer, a portableinformation terminal (mobile computer, portable telephone, PHS (PersonalHandyphone System) etc.) and the like are enumerated.

FIG. 17A shows a mobile computer which is constituted by a main body2001, a camera portion 2002, an image receiving portion 2003, anoperation switch 2004, and a display device 2005. The present inventioncan be applied to the camera portion 2002, the image receiving portion2003, the display device 2005, and the like.

FIG. 17B shows a head mount display which is constituted by a main body2101, a display device 2102, and a band portion 2103. The presentinvention can be applied to the display device 2102.

FIG. 17C shows a portable telephone which is constituted by a main body2201, an audio output portion 2202, an audio input portion 2203, adisplay device 2204, an operation switch 2205, and an antenna 2206. Thepresent invention can be applied to the audio output portion 2202, theaudio input portion 2203, the display device 2204, and the like.

FIG. 17D shows a video camera which is constituted by a main body 2301,a display device 2302, an audio input portion 2303, an operation switch2304, a battery 2305, and an image receiving portion 2306. The presentinvention can be applied to the display device 2302, the audio inputportion 2303, the image receiving portion 2306, and the like.

FIG. 17E shows a rear type projector which is constituted by a main body2401, a light source 2402, a display device 2403, reflectors 2404 and2405, and a screen 2406. The present invention can be applied to thedisplay device 2403.

FIG. 17F shows a front type projector which is constituted by a mainbody 2501, a light source 2502, a display device 2503, an optical system2504, and a screen 2505. The present invention can be applied to thedisplay device 2503.

As described above, the scope of application of the present invention isvery wide, and the present invention can be applied to electronicequipments of any field. Moreover, the present invention can be appliedto any product as long as it requires an electrooptical device or asemiconductor circuit.

According to the present invention, it is possible to form asemiconductor thin film having crystallinity which can be substantiallyregarded as single crystal by forming an amorphous semiconductor thinfilm on an insulator disclosed in the present specification and bycrystallizing the film. By using such a semiconductor thin film, it ispossible to realize a TFT having high performance comparable with orsuperior to a MOSFET manufactured on single crystal.

A semiconductor circuit and an electrooptical device constituted byusing TFTs as described above, and an electronic equipment provided withthose components have extremely high performance, and becomes anextremely excellent product in functionality, portability, andreliability.

1. A method for forming a semiconductor device comprising: flattening aninsulating layer; forming a semiconductor film comprising silicon overthe flattened insulating layer; providing the semiconductor filmcomprising silicon with an element for promoting crystallization ofsilicon; crystallizing the semiconductor film comprising siliconprovided with the element for promoting crystallization of silicon; andsubsequently subjecting the semiconductor film comprising silicon toheat treatment to reduce the element for promoting crystallization ofsilicon in the semiconductor film comprising silicon and to form a firstpart of the semiconductor film comprising silicon to become an activelayer of the semiconductor device and to form a thermal oxidation filmover the first part to become the active layer by thermal oxidation of asecond part of the semiconductor film comprising silicon, wherein thefirst part of the semiconductor film comprising silicon has a finalthickness of the active layer of the semiconductor device from 10 nm to75 nm.
 2. A method according to claim 1 wherein the element forpromoting crystallization of silicon is selected from the groupconsisting of nickel, cobalt, iron, palladium, platinum, copper, gold,germanium, lead and indium.
 3. A method according to claim 1 wherein theheat treatment is carried out in an atmosphere containing halogenelement.
 4. A method according to claim 3 wherein the atmosphere furthercontains oxygen.
 5. A method according to claim 1 further comprisingforming a gate insulating film over the semiconductor film comprisingsilicon, and the heat treatment is carried out after the formation ofthe gate insulating film.
 6. A method according to claim 5 wherein thegate insulating film comprises a material selected from the groupconsisting of silicon oxide, silicon nitride and silicon nitride oxide.7. A method according to claim 1 wherein the semiconductor device isincorporated into one selected from the group consisting of a computer,a head mount display, a portable telephone, a camera, a rear typeprojector and a front type projector.
 8. A method for forming asemiconductor device comprising: flattening a thermal oxidation film;forming a semiconductor film comprising silicon over the flattenedthermal oxidation film; providing the semiconductor film comprisingsilicon with an element for promoting crystallization of silicon;crystallizing the semiconductor film comprising silicon provided withthe element for promoting crystallization of silicon; and subsequentlysubjecting the semiconductor film comprising silicon to heat treatmentto reduce the element for promoting crystallization of silicon in thesemiconductor film comprising silicon and to form a first part of thesemiconductor film comprising silicon to become an active layer of thesemiconductor device and to form a thermal oxidation film over the firstpart to become the active layer by thermal oxidation of a second part ofthe semiconductor film comprising silicon, wherein the first part of thesemiconductor film comprising silicon has a final thickness of theactive layer of the semiconductor device from 10 nm to 75 nm.
 9. Amethod according to claim 8 wherein the element for promotingcrystallization of silicon is selected from the group consisting ofnickel, cobalt, iron, palladium, platinum, copper, gold, germanium, leadand indium.
 10. A method according to claim 8 wherein the heat treatmentis carried out in an atmosphere containing halogen element.
 11. A methodaccording to claim 10 wherein the atmosphere further contains oxygen.12. A method according to claim 8 further comprising forming a gateinsulating film over the semiconductor film comprising silicon, and theheat treatment is carried out after the formation of the gate insulatingfilm.
 13. A method according to claim 12 wherein the gate insulatingfilm comprises a material selected from the group consisting of siliconoxide, silicon nitride and silicon nitride oxide.
 14. A method accordingto claim 8 wherein the semiconductor device is incorporated into oneselected from the group consisting of a computer, a head mount display,a portable telephone, a camera, a rear type projector and a front typeprojector.
 15. A method for forming a semiconductor device comprising:flattening an insulating layer; forming a semiconductor film comprisingsilicon over the flattened insulating layer; providing the semiconductorfilm comprising silicon with an element for promoting crystallization ofsilicon; crystallizing the semiconductor film comprising siliconprovided with the element for promoting crystallization of silicon;subsequently subjecting the semiconductor film comprising silicon toheat treatment to reduce the element for promoting crystallization ofsilicon in the semiconductor film comprising silicon and to form a firstpart of the semiconductor film comprising silicon to become an activelayer of the semiconductor device and to form a thermal oxidation filmover the first part to become the active layer by thermal oxidation of asecond part of the semiconductor film comprising silicon; introducing animpurity element for giving one conductivity into the first part of thesemiconductor film comprising silicon; and activating the impurityelement, wherein the first part of the semiconductor film comprisingsilicon has a final thickness of the active layer of the semiconductordevice from 10 nm to 75 nm.
 16. A method according to claim 15 whereinthe element for promoting crystallization of silicon is selected fromthe group consisting of nickel, cobalt, iron, palladium, platinum,copper, gold, germanium, lead and indium.
 17. A method according toclaim 15 wherein the heat treatment is carried out in an atmospherecontaining halogen element.
 18. A method according to claim 17 whereinthe atmosphere further contains oxygen.
 19. A method according to claim15 further comprising forming a gate insulating film over thesemiconductor film comprising silicon, and the heat treatment is carriedout after the formation of the gate insulating film.
 20. A methodaccording to claim 19 wherein the gate insulating film comprises amaterial selected from the group consisting of silicon oxide, siliconnitride and silicon nitride oxide.
 21. A method according to claim 15wherein the semiconductor device is incorporated into one selected fromthe group consisting of a computer, a head mount display, a portabletelephone, a camera, a rear type projector and a front type projector.22. A method according to claim 15 wherein the activation is carried outby furnace annealing.
 23. A method according to claim 15 wherein theactivation is carried out by laser annealing.
 24. A method according toclaim 15 wherein the activation is carried out by lamp annealing.